I've been reading alot about plenums and runner lengths but I can't find any formulas or calulations for the kind of setup the rx7 has.
My 2 questions are:
1) How do figure in the length and size of the secondary runners?
2) How do you size the plenum?

I was thinking if using the calculators and whatnot for runners and plenums for the primary and the just have the secondary runners as short and direct as possible.

 

these are both from the SAE paper "Improvement of rotary engine performance by new induction system" 831010, basically its the evolution from the 4 port 12A/6 port 12A into the 13B GSL-SE engine.

1. Mazda tried a bunch of different diameters, they chose the same one you would have.


2. again they tried different sizes. the dynamic effect of the intake gets more pronounced when the plenum is smaller, so VE goes up. you can see that the GSL-SE has the biggest one, and they keep getting smaller, until the Rx8 which basically doesn't have one.



with length, it seems like they cut and try.... the GSL-SE and S4 are 500mm, the S5 is 600 and 400?, the Rx8 actually has different lengths for each port.